This application is directed in general to imaging using an ultrasound machine. This application is directed in particular to tissue harmonic imaging using an ultrasound machine.
Tissue harmonic imaging is a known imaging method used in ultrasound machines. Such imaging was started based on a perceived need to improve the effectiveness of acoustic contrast agents, where the backscattered signal is rich in harmonics. Once introduced into clinical practice, it became obvious that images could be obtained without the introduction of contrast, and that furthermore these images demonstrated gains in image clarity. Tissue harmonic imaging was therefore established as an imaging mode in its own right. Previously, two methods were generally used in generating tissue harmonic imaging, such methods comprising single firing band pass filtering and multiple firing pulse/phase inversion.
In a known single firing method, the harmonic components are extracted by applying a band pass filter to the received signal. However, using band pass filtering may limit the bandwidth. For example, the bandwidth of the transmit signal and the band pass filter must be limited to separate the fundamental spectrum and harmonic bands. A filter cutoff must be selected, representing a trade off between loss of harmonic signal and contamination from the fundamental spectrum. The limitation of the probe bandwidth will force the use of a narrowed transmit fundamental band. A narrow bandwidth may require lengthening the transmit pulse and filter impulse response, which, in turn, may result in a degradation of axial resolution.
The limitations of known single firing harmonic band pass filtering has been largely overcome using pulse (phase) inversion as disclosed in U.S. Pat. No. 5,706,819 issued Jan. 13, 1998. Known phase inversion method uses two or more sequential pulses, directed along the same path but with the inverted polarities. The coherent sum of the backscattered signal resulting from these firings eliminates the odd harmonics (including the fundamental) while keeping the even harmonics (including the second harmonic) used to form images. Phase inversion imaging enables broadband pulses, thus spatial resolution to be retained for harmonic imaging. However, for regular pulses, broadband means short pulse length, which in turn, results in a loss of penetration especially in harmonic imaging where the second harmonic signal is about 20 dB less than the fundamental signal. Further, requirement of double or multiple firing along the same beam path reduce the frame rate in phase inversion case.
It is advantageous to have the resolution associated with pulse inversion harmonic imaging while keeping the frame rate, penetration and signal to noise ratio (alternatively referred to as “SNR”) associated with single firing harmonic imaging. However, several major challenges are contemplated. First, it is always a challenge to obtain sufficient penetration and improved SNR while maintaining good resolution. For fundamental imaging, frequency modulated signals (e.g., chirp) have been employed with high time-bandwidth product. This approach may result in greater penetration and improved SNR with the same bandwidth as conventional pulses after proper decoding to avoid significant range lobes and maintain good axial resolution. In U.S. Pat. No. 6,213,947B1 issued Apr. 10, 2001, discloses using of a matched filter which is designed for the highest SNR achievement for frequency/“nonlinear phase” modulated coded excitation. The decoding filter may be applied to RF signals, which have a very high sampling frequency, or to demodulated RF signals using complex filter coefficients. In both cases, the decoding filter is quite large and costly. The cost issue prevents most of the current ultrasound companies from implementing the frequency/nonlinear phase modulated coded excitation even in their premium ultrasound machines. Even if the cost were acceptable, the decoding filter with a matched design may not work as planned most of the time, especially for harmonic imaging. This is due to the fact that in harmonic imaging, the decompressing becomes more difficult since the phases change twice as fast, as a function of time, compared to the fundamental, and the complexity of the generation of tissue harmonic through generally heterogonous tissue severely degrade the effectiveness of the matched compression filtering, which is always designed based on ideal situations. As a result, the range side lobe level could be very high, which does not make sense for practical applications of ultrasound imaging. A mismatched filter (similar to that disclosed by T. X. Misaridis and J. A. Jensen, in the paper titled An Effective Coded Excitation Scheme Based On A Predistorted FM Signal And An Optimized Digital Filter), which targets on decreasing the range side lobe level with some sacrificing of SNR, may be helpful for decoding.
Another challenge in harmonic imaging is the near field harmonic performance. Harmonic signals in tissue are different from local nonlinearity of micro bubbles as the tissue harmonic signals are generated gradually during propagation. Thus, for very near field (e.g., less than about 2 cm) there may not be enough harmonic components generated in tissue. Generally, this causes blooming-like image characteristics where a thin line could appear as a thick block due to either saturation of the over-gained small second harmonic signal or leakage of the fundamental signal especially in high frequency case. This phenomenon seriously limits the application of harmonics in near field structure such as small parts and superficial structure with high frequency probes.
The third challenge is the frame rate. Phase inversion techniques generally sacrifice frame rate since multiple firings are required per beam position. Several patents disclose frame rate improvement, for example U.S. Pat. No. 6,436,046 B1 issued Aug. 20, 2002 and U.S. Pat. No. 6,066,099 issued May 23, 2000. These patent documents include, for example, the multi-line acquisition (a plurality of receive beams are associated with one of the transmit beams), spatially adjacent transmission of phase inverted vectors, and simultaneously multiline transmission.
Thus there exists a need to provide high quality harmonic imaging capability throughout the field of view, in a way that solves these challenges.